The present invention relates generally to electrolytes suitable for use in a lithium ion cell or battery and relates more particularly to a novel electrolyte suitable for use in a lithium ion cell or battery.
Growing demands for lightweight, portable, rechargeable batteries have generated a need for rechargeable lithium ion batteries with higher performance, longer life, improved safety and a wide operating temperature range of −40° C. to +70° C. Current lithium ion battery technology is limited by the deterioration of the battery electrolyte and the electrolyte-electrode interface as a result of elevated operating and storage temperatures. Traces of water, alcohols or other protic solvents can elevated to pressure build-up from hydrogen generation (see McDonald, “Sources of Pressure in Lithium Thionyl Chloride Batteries,” J. Electrochem. Soc., 129(11):2453-7 (1982), which is incorporated herein by reference). Most of the more common electrolytes use flammable solvents with low flash-points. Operation at low temperatures is limited by diminishing conductivity and phase separation, which can lead to electrode and electrolyte deterioration during extended cycling.
Unfortunately, the higher conductivities for common non-aqueous, polar co-solvents used in electrolytes for lithium ion batteries tend to be derived from solvent systems which include straight chain or cyclic ethers, which have very low flash points and which can severely limit the upper temperature for storage and operation of the battery. The use of cyclic carbonates like propylene carbonate (PC) and ethylene carbonate (EC) improves the situation with higher dielectric constants and flashpoints, but the low-temperature conductivity for most of the aprotic, organic solvents studied so far severely limits power density. In addition, low flash point solvents can contribute to fire hazards associated with equipment failure in manned flights. One of the best low-temperature electrolytes examined so far consists of a 3:1 mix of methyl formate and ethylene carbonate with LiAsF6, with a conductivity of 0.0084 S/cm reported at −40° C. (see Ein-Eli et al., “Li-Ion Battery Electrolyte Formulated for Low-Temperature Applications,” J. Electrochem. Soc., 144(3):823-9 (1997), which is incorporated herein by reference). This conductivity approaches that measured for some very conductive inorganic electrolytes, like 1.0M LiAlCl4 in thionyl chloride (SOCl2), which is 0.013 S/cm at −40° C. (see McDonald et al., “Low Temperature Characteristics of Lithium/Thionyl Chloride Cells, Progress in Batteries & Solar Cells, 5:294-8 (1984), which is incorporated herein by reference).
Considerable effort has been expended over the last 40 years to develop electrolytes for lithium ion batteries using lithium salts in various aprotic solvents. Conductivities as high as 20 mS/cm2 have been measured for inorganic electrolytes like LiAlCl4 in thionyl chloride, but these are not suitable for rechargeable batteries. Organic solvents are preferred for rechargeable batteries; however, most of the solvents with good lithium ion mobility have low flash points or have too narrow an electrochemical window of stability. This stability is essential to avoid oxidative or reductive decomposition of the material on the electrode surfaces during the battery charging and deep discharging. Higher electrolyte conductivities decrease battery internal resistance, increasing available power, while higher flashpoints decrease the risk of fire in a battery if exposed to excessive heat. Table I lists some of the properties for certain liquid electrolyte systems.
TABLE 1DipoleFreezingBoilingmomentPointPointFlashpointAcceptorCompoundFormula(Debye)(° C.)(° C.)(° C.)No.Butylene Carbonate5.28−45  2511350.23 Propylene Carbonate4.98−48.82421310.32 Ethylene Carbonate4.8 ca 202481600.32
Unfortunately, most liquid electrolyte systems can be considered flammable, with the open chain ether types somewhat more flammable that the organic carbonates. In addition, electrolytes using lithium perchlorate, LiClO4, are rarely used because of the potential for rapid exothermic decomposition under certain conditions. The use of lithium hexafluoroarsente, LiAsF6, is also avoided because of concerns about arsenic toxicity in consumer products. Most commercial lithium batteries now use lithium hexafluorophosphate, LiPF6, which has reasonable stability and environmental compatibility.
In “LiBOB and Its Derivatives: Weakly Coordinating Anions, and the Exceptional Conductivity of Their Nonaqueous Solutions,” Electrochemical and Solid State Letters, 4(1):E1-E4 (2001), which is incorporated by reference, Xu et al. reviewed a number of electrolyte salts which showed good conductivity down to −40° C., but had to choose the highly flammable solvent dimethoxyethane (DME). In practical battery electrolytes, combinations of solvents are often used to provide a wide operating temperature range, maximum ionic conductivity and the desired cathodic stability. In addition, electrolytes must provide a protective passivation film on the anode during early charge cycles, in order to limit continued anodic decomposition of the solvent. Their conductivities tend to be somewhat lower than the pure solvents, used alone.
Other documents of interest include Xu et al., “Sulfone-Based Electrolytes for Lithium-Ion Batteries,” Journal of The Electrochemical Society, 149(7):A920-6 (2002), and Smart et al., “Electrolytes for Low-Temperature Lithium Batteries Based on Ternary Mixtures of Aliphatic Carbonates,” Journal of The Electrochemical Society, 146(2):486-92 (1999), both of which are incorporated herein by reference.